Update README.md to include local installation instructions

[fozcodes]

Hey there!

I could actually never get roarr to work as a global install. I use asdf as a manager and it would always fail saying that no node version was declared for roarr. This was probably a shim problem and/or a Fish Shell problem. I either case, I'm not super crazy about installing anything globally.

This PR just adds an explanation for installing as a project dependency. Works great, even with nodemon and all the other logs in our app.

Cheers.

[coveralls]

Pull Request Test Coverage Report for Build 41

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 39:	0.0%
Covered Lines:	1
Relevant Lines:	1

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 41

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 39:	0.0%
Covered Lines:	1
Relevant Lines:	1

---

💛 - Coveralls

[gajus]

This is basic knowledge that an avg. Node.js developer would know. Not worth crowding the documentation.